1. Field of the Invention
The present invention relates to an iron-based power mixture for powder metallurgy comprising a lubricant which does not contain any metal components, and exhibiting excellent flowability and property that the mixture is smoothly discharged from a hopper.
2. Related Art
An iron-based powder mixture for powder metallurgy is in general prepared by mixing powder for alloy, such as copper powder, graphite powder or iron phosphide powder and a lubricant such as zinc stearate with iron-based powder as a base, and optionally adding powder f or improving cutting property into the resultant.
However, such a mixture has the following drawbacks.
The first drawback is that such a mixture easily segregates in terms of its particle size, its particle shape and its chemical composition. In other words, the mixture contains plural sorts of powders different in their size, shape and density, and consequently the powders are not distributed uniformly in the mixture when the mixture is transported, put into a hopper or taken out from it after the mixing, or the mixture is charged into a mold and then subjected to press-molding process. Thus, the aforementioned segregation is caused.
As regards, for example, the mixture of iron powder and graphite powder, in transportation by truck, the iron powder and the graphite powder are freely moved and shifted in a transporting container by vibration, so that they will not be distributed uniformly. Thus, segregation is caused. It is especially well-known that the graphite powder having a small density rises to the surface of the mixture. As is also well known, in the mixture put into a hopper, segregation is caused by movement thereof in the hopper, and thus the concentration of the graphite powder in the mixture discharged from the hopper varies among the early stage, the middle stage and the late stage of the discharging.
When the mixture wherein the segregation is caused is press (compression)-molded and then the resultant molded bodies are sintered to prepare sintered bodies as final products, the composition, size and strength of the sintered bodies vary among the respective products. Thus, inferior products are prepared. All of the mixed copper powder, graphite powder and iron phosphide powder have a small particle size than that of iron powder; therefore, the specific area of the mixture is increased so that its flowability drops. The drop in the flowability of the mixture results in the drop in the speed of charging the mixture into a mold. As a result, the productivity of molded bodies, which may be called green compacts as the case may be, is lowered.
To prevent segregation of such a mixture, Japanese Patent Publication (JP-B) No. 5-27682 discloses an improved powder composition comprising an iron based powder, at least one powder for alloy, and a binder for the iron-based powder and the powder for alloy, and prepared by mechanically mixing the binder with the iron-based powder and the powder for alloy, characterized in that the binder is methacrylic polymer which is substantially water-insoluble and the binder is contained in an amount of about 0.005-1.0% by weight. The column of "Detailed explanation of the inventions" in this publication discloses a homopolymer of methylmethacrylate (poly(methylmethacrylate)) as a methacrylic polymer. However, this mixture disclosed in JP-B-No. 5-27682 contains methyl polymethacrylate as the binder for bonding the powder for alloy to the iron-based powder but not as powder. Methyl polymethacrylate as the binder naturally causes segregation to be prevented, but property of causing the mixture to be smoothly discharged from any hopper, which will be referred to as the hopper discharging property hereinafter, is insufficient. Thus, it needs to be improved. There also remains another problem that the resin as the binder is dissolved in an organic solvent and then the resultant is applied to the surface of the iron-based powder thinly so that the cost for collecting the organic solvent will be raised.
As another segregation preventing technique, Japanese Patent Application Laid-Open (JP-A) No. 1-219101 discloses a mixture powder wherein graphite powder is bonded to the surface of iron-based powder with zinc stearate as a binder. Furthermore, the present inventors proposed a method wherein a metallic soap and a fatty acid are used as a binder in JP-A-No. 3-162502. However, the mixtures disclosed in these publications contain compounds containing zinc or other metal elements as the binder. Thus, the metal elements in the binder are turned into oxides during sintering a molded body, so that the oxides will cause contamination inside a sintering furnace and change in the chemical composition of the sintered body.
To overcome these problems, mixtures wherein a binder containing no metal element is used are proposed in JP-A-No. 60-502158 and JP-A-2-217403. In the techniques described in these publications, however, zinc stearate is finally added as a lubricant since the binder itself does not have any lubricating effect. Thus, the problem about the aforementioned hopper discharging property is not overcome.
In the light of these problems, the present inventors disclosed a solution for these problems in JP-A-No. 5-148505, that is, an iron-based mixture for powder metallurgy prepared by mixing a powder for alloy and an iron-based powder with at least one binder selected from the group consisting of stearatic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenebis(stearamide), and ethylenebis(stearamide), heating and subsequent cooling the resultant so that the powder for alloy and the like are bonded to the iron-based powder with the hardened binder, and adding into this the same sort of powder as the binder and a powder such as zinc stearate, as a lubricant, and mixing the resultant; and a method for preparing this mixture. The inventors proposed that lithium stearate should be used as the lubricant in the case of using hot-melted zinc stearate, higher fatty acid, amide of higher fatty acid, or wax, as the binder.
Even by this technique, however, the contamination inside the sintering surface cannot be completely eliminated since the mixture still contains a metallic soap such as zinc stearate or lithium stearate in a small amount. In the case of using a lubricant containing no metallic soap such as zinc stearate or lithium stearate, that is, a lubricant consisting of only organic substances without any metal component being contained, the flowability and the hopper discharging property of the obtained iron-based powder mixture are deteriorated.